Various package cracks observed in service, with different locations and propagation directions in plastic package of surface mounting type are analyzed in terms of the energy release rate in fracture mechanics. It has been reported that the types of the cracks observed in the package depend on the chip size, relative thickness of epoxy resin on the chip and under the chip pad, material properties and the adhesion strength of the interfaces in the package. The stress derived from the thermal expansion mismatch of the materials during the temperature cycling and the pressure induced by the expansion of the moisture absorbed in package at soldering temperature are considered separately in estimating the effects of various parameters on the package cracks. The influences of the stresses derived from the thermal expansion mismatch of the materials during the temperature cycling and during the solder reflow process on the package crack are also compared. As a result of the analysis, we find that the propagation direction of the package crack is dependent on the location of the delaminated interface from which the package crack originate and the loading type imposed on the package, and that the influences of various parameters on the package cracks are largely dependent on the loading type.